Customize Some Opencart Feature

We need someone to customize some features in our Opencart website. This will include customizing a module call Admin Order Edit. [login to view URL] By default this module allows us to edit order by adding/removing product, however the inventory quantity is not taking effect. So you'll need to enable this. Also, we need the old invoice to maintain in the system and generate a new invoice upon each Order Edit, so you will create a new tab in Order Details to store and display all older invoices. Your customization should not affect any other features and must be compatible with other modules that are currently in the system. Kindly download and read thoroughly the attached document on the requirements. Ignore the second requirements on GST for now, that will be customized by another developer, but your customizations must work hand-in-hand with it.